1. Field of Invention
The present invention relates to a method and control system for controlling pasteurization of in-shell eggs in a fluid.
2. Description of Related Art
The United States Department of Agriculture (USDA) regulates minimum safety standards for pasteurizing in-shell eggs. These standards are promulgated in order to ensure that certain microorganisms, including such infectious organisms as Salmonella, are substantially destroyed prior to distribution and consumption of the eggs. The USDA defines pasteurization as a heat treatment for the purpose of killing these disease causing organisms.
One source of infection arises when the egg shells come into contact with organic refuse. Contamination results because the egg shells have numerous pores which permit infectious microbes, which are contained in the organic refuse, to penetrate the pores of the eggs. Another source of infection results from trans-ovarian contamination. This occurs when chickens or other poultry ingest or are otherwise infected by infectious microbes and transfer the microorganisms directly into the eggs.
The Nutrition Action Health Letter published by the Center for Science and the Public Interest (July/August 1991 edition, Vol. 18, No. 6, "Name Your (Food) Poison") reports that in-shell eggs are particularly difficult to pasteurize because of their structure. In addition, this article reveals that one in ten thousand eggs is contaminated with salmonella enteritis.
Techniques for improving pasteurization of eggs have been proposed. These techniques attempt to destroy infectious disease causing organisms in in-shell eggs without substantial loss of functionality. One approach to pasteurizing in-shell eggs involves heating the in-shell eggs in water baths, for various times and at various temperatures. The time/temperature ratios vary widely because different approaches involve a compromise between the degree of safety achieved and the quality or the functionality of the eggs retained after pasteurization is completed. The USDA has devised time/temperature ratios, but they are only for liquid eggs.
Cox et al. (PCT/US94/12950) discloses a method for destroying infectious disease causing organisms in in-shell eggs without substantial loss of functionality. Cox et al. employs a temperature versus time relationship in order to accomplish pasteurization of the in-shell eggs. An initial egg temperature and processing temperature at the beginning of the pasteurizing process of a whole shell egg must be known. These temperatures are used to determine the total processing time, e.g., the total length of time over which the eggs are heated. According to a preferred embodiment of Cox et al., minimum temperatures/time requirements for liquid whole eggs are applied equivalently to in-shell eggs once the selected pasteurization temperature has been achieved at the shell egg yolk center.
Cox et al. uses the following temperature time table for determining the pasteurization time of in-shell eggs.
______________________________________ Temperature Real Processing Time (RPT) (Minutes) ______________________________________ 130.degree. F. =65 131.degree. F. =49 132.degree. F. =38 133.degree. F. =28 134.degree. F. =20 135.degree. F. =16 136.degree. F. =11 137.degree. F. =8 138.degree. F. =6 139.degree. F. =4.75 140.degree. F. =3.5 ______________________________________
This table describes the processing of in-shell eggs after they attain the required pasteurizing preprocessing temperature. The initial temperature is applied until the in-shell eggs reach a temperature equilibrium with the heat transfer medium. The RPT for a given pasteurization regimen can only begin after this point has been reached.
Cox et al. also discloses that factors including the size and internal initial temperature of the eggs may affect the time required for the eggs to reach the effective processing temperature. Thus, an initial temperature that causes pasteurization of one batch of eggs may result in impaired functionality of a second batch of eggs having a smaller size, depending on the variables associated with that particular batch of eggs.
Davidson International Application No. PCT/US96/13006 (U.S. application Ser. No. 08/519,184), also discloses methods to pasteurize in-shell eggs using time/temperature relationships. In particular, Davidson discloses heating a yolk of the egg to within the range of 128.degree. F. to 138.5.degree. F. Once the yolk reaches this temperature range, it must be maintained at this temperature range for a certain time and within certain parameters.
FIG. 1 shows a temperature versus time curve implemented by the Davidson system. This curve is based substantially on the data of the above table. Referring to FIG. 1, the temperature of the egg yolk must be maintained between parameter line A and parameter line B in order for sufficient pasteurization to occur. According to Davidson, this will reduce the Salmonella by at least 5 logs, while at the same time retaining the functionality of the eggs. If the eggs are heated to a limit outside parameter lines A and B, however, the eggs will either lose their functionality or remain insufficiently pasteurized. Thus, according to Davidson it is imperative that the temperature of this system stay within the predefined parameters.
Factors such as loss of water, temperature overshoot (e.g. raising the temperature too high), inefficient temperature sensors (e.g. low response time for raising the temperature to a predefined temperature range), and numerous other factors make it possible for the bath temperature to stray from preferred parameters. The size of the eggs, the number of eggs placed in the bath and the initial internal temperature of the eggs will also affect the pasteurization time and functionality of the eggs.